After a 17-year absence, cicadas from Brood XIII are about to emerge in Illinois, Indiana, Iowa, Michigan and Wisconsin.

Facts and misconceptions about the insects:

Q: Why do they disappear for 13 or 17 years?

A: Scientists aren't sure why, but some believe their synchronized absences are meant to fool predators. Others believe climate history or glacial cycles are responsible.

Q: Why do periodical cicadas appear only in the eastern half of the USA?

A: They feed on sap from deciduous trees those that lose their leaves in the fall. There are fewer such trees in the West.

Q: Is cicada another name for locust?

A: No. Locusts are a kind of grasshopper. They eat plants.

Q: Why do cicadas make such a racket?

A: The "chorus" is made by males to attract females. The males contract tymbals, or ridged membranes, on their abdomens to make the sound, which is amplified by their almost-hollow abdomens.

Each species has its own unique sound, and the chorus can reach 90 decibels as loud as a lawnmower.

Q: Do cicadas bite or sting?

A: No. They're harmless, though males make a loud buzz when they're alarmed, and they're big enough to startle humans in return: The largest can have 3-inch wing spans. Young, small trees can be damaged when females deposit their eggs inside branches.

Q: Why do they have red eyes?

A: "To make them look more ferocious," says Dan Summers, collections manager of the Field Museum's insect division.

CHICAGO  The cicadas are out there already. Millions of them are just below the ground, silent for now, their wings still undeveloped. They're moving toward the surface, pushing soil up into little turrets.

Any day now, when the soil warms to 64 or 65 degrees, the big, red-eyed bugs will end their 17-year hiatus here and emerge simultaneously in a process that's still a scientific mystery.

The nymphs will crawl up trees, shed their skin and stretch their wings. The males will begin their deafening chorus to attract females. They will mate, and then the females will gouge holes into branches with swordlike appendages called ovipositors and deposit their eggs.

Four to six weeks later, newborn nymphs will fall to the ground and burrow for another 17-year stay. The adults will die. By August, it will all be over.

The return of cicadas in the upper Midwest means scientists such as David Marshall from the University of Connecticut will travel here to study the baffling bugs. Different broods, or groups, of cicadas emerge in different parts of the eastern half of the USA in different years. Other species of cicadas show up every summer.

Dan Mozgai, an aficionado from Metuchen, N.J., founder of the website cicadamania.com, is coming just for the spectacle.

And Amy Carlson will marry Doug Kieso outdoors at a wooded estate in suburban Chicago on June 16. Their guests will be given cicada-shaped cookies to commemorate the inevitable uninvited insect visitors.

Carlson, 39, who works for an insurance company, says her brother gave her good advice: "You can really let it get to you and be upset, or you can just show people how easygoing you are and let it go." Her fiancé and her daughter Lindsey Carlson, 15, don't want to move the wedding and besides, Carlson says, "Bugs don't bother me."

Dan Summers is fascinated by cicadas. He's the collections manager of the insect division at Chicago's Field Museum and stops at forest preserves near the city every night to check on the nymphs' progress.

Summers is answering lots of phone calls, too: "Some people want to know the exact dates so they can leave town, and some want to know the dates so they can come to town." The Field Museum's cicada exhibit opens Friday.

Scientists have identified 15 broods of "periodical" cicadas — those that emerge every 17 or 13 years. Brood XIII is the one about to arrive here. Brood X hit the East Coast and parts of the Midwest in 2004; next year, Brood XIV will appear.

There are theories, but no one has figured out why periodical cicadas emerge in such large numbers or why their life cycles have 13- and 17-year patterns, Marshall says.

It was long thought that they disappeared for so long so predators couldn't count on them being around every year, and that they return in huge numbers so birds, snakes, foxes and other animals can't possibly eat them all.

More recent research, Marshall says, suggests that the long life cycles evolved because of glacial periods that sometimes resulted in successive years of chilly summers. During this emergence, he'll study the outer boundary of the brood.

He got hooked on cicadas in graduate school. "I became kind of captivated by this concept of the long, prime-number life cycle and the strange evolutionary puzzle they present," Marshall says.

Mozgai, 38, says he isn't really "a bug person," but his passion for cicadas began when he was a guest at a 1996 wedding that was overtaken by them. He and a dozen other "hard-core cicada maniacs" take photos and videos of emergences and discuss them online.

He usually doesn't tell people about his hobby, says Mozgai, who manages Web developers for a telecommunications company: "I don't always share the information on a first date." He'll be in Chicago in early June, he says, not to do research on the emergence, but just because "I want to see it."

Summers says cicadas are tasty, with the texture of avocados and a nice crunch. "You can parboil and freeze them, cook them on skewers or put them in chili or pizza," he says. "Invite your friends over and wash them down with beer."

Marshall, on the other hand, does not eat them. "Um, no," he says.

Most people find the critters annoying, even though they don't bite or sting. Summers recommends that young trees be wrapped in cheesecloth to protect their branches from being damaged by too many eggs; larger trees are probably safe.

Nick Tsoukalas, general manager of Athena Greek Restaurant, which has outdoor seating for 200, says he and his customers will just have to put up with them: "I've got no choice."

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Cicadas made their mark on the East Coast and parts of the Midwest in 2004.

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